I would use some soapy water and try to determine the region of the leak. I would put in an ounce or so of Stan's, blow up the tire and give the wheel a good spin. I suspect that sort of slow leak will seal easily. Also, first be sure to look around the tire for flints. If the tire is retaining any glass, that slow leak might just get a bit faster, so you want to take the opportunity to remove any injuring objects from the tread.

Something also to bear in mind, that soapy water test might indicate a leaky valve stem, so you should check the core to be sure there's no garbage embedded there.